The present invention relates to metal masses adapted for internal oxidation to generate dispersion strengthening by the in situ formation of hard, refractory oxide phases therein, and a process for dispersion strengthening which utilizes said metal masses.
In the past, it has been recognized that strength and hardness can be imparted to a solid solution alloy of ductile matrix metal, and a solute metal, the matrix metal having relatively low negative free energy of oxide formation, and the solute metal having a relatively high negative free energy of oxide formation. A substantial difference in negative free energy of oxide formation between matrix metal and solute metal is essential here. The negative free energy of oxide formation is a measure of the ease with which a metal will oxidize. A metal with a low negative free energy of oxide formation is difficult to oxidize, and a metal with a high negative free energy of oxide formation is easy to oxidize. By heating the alloy under oxidizing conditions, the solute metal preferentially oxidizes to cause the in situ formation of hard, refractory oxide particles in the matrix metal, substantially wlthout oxidizing the matrix metal. This technique is known as the in situ internal oxidation of the solute metal to the solute metal oxide, or simply "internal oxidation". In internal oxidation, the matrix metal is relatively noble compared to the solute metal so that the solute metal will be preferentially oxidized.
The hard, refractory metal oxides formed in the matrix metal cause the alloy to be dispersion strengthened. Dispersion strengthening imparts to these materials a high strength, a high electrical conductivity and a high heat resistance. Dispersion strengthened metal products, such as copper dispersion strengthened with aluminum oxide, have many commercial and industrial uses where high temperature strength, high electrical conductivity, and/or heat conductivity are desired or required. Such uses include frictional brake parts such as linings, facings, drums, and the like and other machine parts for frictional applications, contact points for resistance welding electrodes, electrodes generally, electrical switches and electrical switch gear, transistor assemblies, wires for solderless connections, wires for electrical motors, lamp leads, and many other related applications. The metal masses of the present invention are useful in the production of dispersion strengthened products for the above and other applications.
In the past, attempts have been made to dispersion strengthen alloys by various methods of internal oxidation. These methods may be divided into two categories. The first category is powder metallurgical processes. The second category is internal oxidation of bulk alloy masses. In the first category, U.S. Pat. No. 3,026,200 shows the surface oxidation of alloy powder followed by a heat treatment in an inert atmosphere to diffuse oxygen from the surface of the alloy to preferentially oxidize the solute metal to solute metal oxide within the alloy powder.
U.S. Pat. No. 3,184,835 discloses the internal oxidation of copper-beryllium or copper-aluminum alloys wherein the oxidant is a sintered and milled mixture comprising about 50% copper oxide and about 50% aluminum oxide. The sintered oxidant residue is physically separated from the internally oxidized alloy powder before the powder is formed into dispersion strengthened metal products. The use of this sintered mixture as the oxidant is said to minimize adhesion of the oxidant residue to the internally oxidized alloy.
U.S. Pat. No. 3,779,714 discloses a process for the internal oxidation of alloy powders, for instance copper-aluminum alloy powders, wherein the oxidant is a blend of a heat-reducible metal oxide and a finely divided hard, refractory oxide. This oxidant blend is disposed such that after oxidation of the alloy powder is accomplished, the oxidant residue has substantially the same net composition as the internally oxidized alloy powder, and thus does not have to be removed.
In each of these cases, dispersion strengthened alloy powders are produced before any shapes or articles are attempted to be made from the powders. Such forming procedures after internal oxidation have proven to be very difficult because of the high strength of the dispersion strengthened alloy powders.
In the second category bulk alloy masses are internally oxidized. U.S. Pat. No. 3,399,086 discloses the internal oxidation of copper-aluminum alloy in plate or strip form using copper oxide as the oxidant. The shape to be oxidized is packed in the oxidant. The copper oxide is reduced to give up its oxygen for the preferential oxidation of the alloyed aluminum to form interspersed particles of aluminum oxide within the copper matrix. This process is disclosed as often taking several hours, and then, is usually only effective to dispersion strengthen a relatively thin section of the plate or strip surface.
U.S. Pat. No. 3,552,954 discloses a process for the internal oxidation of alloy strips. According to this process, the alloy strip is internally oxidized under a controlled atmosphere and then reduced in the presence of hydrogen to remove excess oxygen. The alloy strip is then pulverized and reformed into the final dispersion strengthened product. In this patent, it is disclosed that the original processed alloy strip was an unsuitable dispersion strengthened product because of excessive hydrogen embrittlement.
U.S. Pat. No. 3,615,899 discloses another method for the bulk internal oxidation of alloy parts. In this process, the alloy part is packed in a cuprous oxide oxidant which contains an inhibitor oxide. The oxidant blend is controlled to provide a maximum internal oxidation velocity. The maximum internal oxidation velocity is said to be required to produce optimum properties in the finished alloy part.
All of the prior art methods in this category dispersion strengthen a bulk alloy part by exposure to an oxidizing environment only at its surface which requires an extremely long period of time and/or the ultimate properties of the alloy are compromised by concomitant reactions which detract from ultimate performance.
One benefit of the present invention is that a cohesive mass of alloy powder and oxidant is formed and at least partial densification takes place before internally oxidizing to dispersion strengthen. Therefore, this forming requires less energy, less force, and produces less die wear than when forming is subsequent to dispersion strengthening.
Another benefit of the present invention is that it produces more intimate contact between the alloy to be internally oxidized and the oxidant. This improved contact decreases internal oxidation time and improves oxygen transfer from oxidant to solute metal in the alloy.
A further benefit derived from the present invention is that a preformed shape can be dispersion strengthened uniformly throughout and in a fraction of the length of time previously required for dispersion strengthening of preformed shapes.